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astroengine writes "Astronomers from the University of British Columbia and University of Edinburgh have created a vast cosmic map revealing an intricate web of dark matter and galaxies spanning a distance of one billion light-years. This is the largest map of its kind and demonstrates that this large-scale web stretches across the universe in all directions. The results of this groundbreaking discovery were presented at the American Astronomical Society conference in Austin, Texas on Monday."

One of the many things that has always bothered me about Dark Matter is how it has gravity but at the same time doesn't seem to be affected by gravity.Case in point, an Astronomer was able to map out the DM field around a pair of colliding galaxies but the DM cloud from one galaxy had just sailed through the DM cloud of the other galaxy without any apparent affect. But at the same time all the visible matter behaved exactly as predicted, swirling together and merging into a larger galaxy.

Once you've come up with a mathematical proof for your demons, you too can be published in Nature or some such.!

I'm working on it now but my retrieval teams keep coming back.... different.... It's so hard to find good help now days that are immune to the effects of non-euclidean space/time. As far as Nature(tm) they have already agreed to publish my work, proof or no. They said something about shifting paradigms and proofs being over rated and not important to their core market group.

Case in point, an Astronomer was able to map out the DM field around a pair of colliding galaxies but the DM cloud from one galaxy had just sailed through the DM cloud of the other galaxy without any apparent affect. But at the same time all the visible matter behaved exactly as predicted, swirling together and merging into a larger galaxy.

That's actually part of why they think it's non-baryonic, IIRC. Normal matter in the colliding galaxies is slowed by friction and electromagnetic forces; the dark matter

Particulate dark matter just never gets too cold to be held by its own gravity. Purely gravitational interactions resulting in ejection of fast-moving particles would eventually let it cool down enough to form planet-sized objects but it'll probably take longer than the lifetime of baryonic matter.

Looking longer and longer by the day. Aether was invented because people felt it SHOULD exist, but expected consequences of it completely failed to show up. Dark matter was invented because there were observations that are very hard to explain any other way and fit increasingly precisely with one another if dark matter is the cause -- there are several different ways of measuring the distribution of dark matter among various clusters of galaxies, and they are giving remarkably consistent answers.

A better example would be phlogiston, which was invented to explain observations, but eventually failed to explain all observations, so it was replaced by a better theory. The same could happen to dark matter, but there are no signs at the moment,

So was Dark Matter. People felt that the Aether should exist because the existing theories at the time governing the physical laws of the universe predicted that it ought to exist. and its nonexistence would mean that those theories were wrong (and they were). What is particularly interesting about proving the non-existance of the Aether (who says you can't prove that something doesn't exist?) is that it was accomplished without adequately forming a

The part you're ignoring is that unlike the aether, there is actual evidence for dark matter, quite a lot of it actually. It's true that at the time it was conceived it was little more than a fudge factor, but that time has long since passed. The Bullet Cluster [wikipedia.org], for example, is probably the stongest single piece of evidence, though by no means the only one. It has a core of regular matter surrounded by a large halo of dark matter which can be observed by measuring the gravitational lensing of light passing through the region.

Fair enough, the same effect could be produced by bending spacetime in some other way, but the only way we know about today is with gravitational mass. Scientists find the assumption that there is a kind of matter we can't see much more readily than they will take the assumption that there is some force other than gravity (or some source of gravity other than mass) that warps spacetime to such a degree over such large volumes of space.

Where do I claim or imply that I have some insight that astrophysicists do not? It is my understanding that the reason we believe in the existence of Dark Matter is because based on our *CURRENT* understanding how the universe works, that is the only thing that we have thought of so far that fits observed phenomonena.

Dark Matter, in this respect, bears a lot of similarity to the idea of the Aether because before the Aether was disproven, observations had already been made that showed that light propagate

People felt that the Aether should exist because the existing theories at the time governing the physical laws of the universe predicted that it ought to exist. and its nonexistence would mean that those theories were wrong (and they were). What is particularly interesting about proving the non-existance of the Aether (who says you can't prove that something doesn't exist?) is that it was accomplished without adequately forming another explanation for what was expected to happen...

In contrast to this, I've read explanations in a number of physics and other scientific textbooks that, strictly speaking, Einstein didn't disprove the existence of the aether at all. His new theory simply ignored the aether. When it turned out that Einstein's equations were better predictors of the universe's behavior than previous equations, physicists didn't insist that there was no aether; they also simply stopped mentioning it. It became irrelevant.

there are several different ways of measuring the distribution of dark matter among various clusters of galaxies, and they are giving remarkably consistent answers.

Those ways (like this one) measure gravitational anomalies. They are not proof that these anomalies are caused by some kind of non-radiating matter.

The main problem with dark matter theories (mind you, there are many of them that each have a different candidate for the role of dark matter), is that they are not full theories. They don't tell anything about how this matter would behave, so no predictions can be based upon them. If some of those theories got refined to the point where a prediction on the dist

Seriously, once you starting talking about vacuum having curvature, geometry, pressure, energy, etc., then you pretty well have gone back to the luminiferous aether hypothesis, just with a bit better math.

...dark matter eventually turns out to be like luminiferous aether from the 19th century? I don't believe anyone has directly observed dark matter.

We don't "directly observe" much of anything. Is that any reason to doubt the existence of x-rays? That our sun is a huge ball of gas undergoing fusion inside? That dinosaurs were actually living creatures?

Even if you run an experiment in your lab, all you "directly observe" is the photons striking your eyes and the sound pressure waves impinging on your ears.

Science is in the business of making inferences from evidence. We have a curious constellation of astronomical/cosmological evidence, for which dark matter is currently the best inference going. Yeah, we may have to throw it out... but the same can be said about *any* conclusion scientists have ever reached.

...dark matter eventually turns out to be like luminiferous aether from the 19th century? I don't believe anyone has directly observed dark matter.

Well, we haven't observed directly very many things, we've just observed side effect [of side effects [of side effects [...]]] of many things we "know" to exist.

I think it's pretty safe to think "dark matter stuff exists", much like it's safe to think "there's stuff inside Jupiter". We don't know what it is, we can only make educated guesses, but we know there must be "something", and we even have pretty tight conditions for what this "something" can or can't be like.

But then, that has been the case since shorty after the invention of fire and the wheel. Seriously, what physical phenomenon are we directly observing? We can't see atoms, we can't even see the circuits making up computers anymore. All we ever did was meassuring the effects of stuff, and conclude that stuff (probably) exists and have certain characteristics.

No, there was no discovery story to miss. We have yet to directly observe dark matter. I'll try an analogy with one caveat: like models, all analogies are wrong. Still, some can be useful...

Picture a ball hanging from a ceiling by an invisible thread. Through various methods you are fairly certain how much the ball weighs, and your knowledge of how gravity works gives you an idea of what it should be doing (i.e. falling), yet it does not. You are faced with two ways to explain this discrepancy: your understanding of gravity is faulty or there is something preventing the ball from falling.

Dark matter is the latter sort of explanation. We think there is a string, and we can infer some of its properties from what we see the ball do but we cannot see it. At the risk of incurring the wrath of cosmologists everywhere I'll give another analogy, even more wrong than the first: one cannot see the air or the winds, but one can deduce their existence from their effects on things one can see.

N.B. The string used in the example above has nothing to do with any of the string theories.

There's a bit more too it than that though, dark matter is no longer just a guess, there is more direct evidence to back it up. To extend your analogy, lets say instead of a single ball hanging you have hundreds of them. A follower of the string theory (pun definitely intended) might make a prediction: some of the balls should have a detectable periodic motion from past disturbances. A thorough survey of the floating balls shows that yes, some of them are swinging like pendulums. It doesn't prove that the balls are hanging from strings, but it means that there's yet another effect that a modified theory of gravity has to take into account, which can be explained very easily by positing the strings. Similarly, there have been a host of indirect observations which show that either there is large amounts of matter that we can't detect or there are dozens and dozens of gravitation effects that are not only not included in current theory but in some cases appear to be mutually exclusive.

Einstein's Telescope, by Evalyn Gates, has a very good entry-level discussion of how gravitational lensing of galactic clusters and superclusters is being used to investigate dark matter. She wrote the book so that it'd be accessible to people without a science background. Friends who aren't scientists have read it and learned a lot from it, although they may have had to read some sections two or three times for some of the concepts to sink in.

If gravitational lensing was how we discover black holes, dark matter, etc... Then why are they not lit up like a floodlight? The lensing that should occur around something that is infinitely dense (Such as a black hole) should also be an infinite amount of lensing, therefore curving all the light in the universe around it, to point back at us. That should (According to my small brain) make black holes the brightest point in any universe.

Black holes are not infinitely dense (caveat, volume may not make too much sense for a black hole if you get pedantic). A black hole with the same mass as the sun would (roughly speaking) be a sphere about 4 miles in diameter. Any light actually hitting that sphere would be absorbed. Light just barely missing it will be bent through large angles. Light passing a few miles away will be bent through smaller angles and so on. If you're in the right place, this can make make something behind the black hole look

They're not infinitely dense. After all, if they were, they would've already sucked the entire universe in. And density may be the wrong way to think about it.

Instead think of them as having high, but not infinite, gravitational fields due to high (not infinite) quantities of mass. That can be achieved with even a "low" density (i.e. mass per volume) - such as galactic superclusters. The amount of bending that occurs when the light passes one of these strong gravitational fields depends on how strong

In a sense, yes. Dark matter was just one hypothesis among many for galaxy rotation speeds before the CMBR studies.

But the CMBR studies were really the "discovery" of dark matter. At the point where the universe took a snapshot of itself, the distribution of matter was still fairly uniform: alternating areas of slightly-denser and slightly-less-dense matter as sound waves rolled through the universe. By measuring the size and magnitude of these compression waves, one thing that we know - by direct observation - is that only 20% or so of matter was interacting with photons, directly or indirectly.

The universe at that time was dominted by 2 forces, gravity and light pressure. Gravity would compress slightly denser patches until light pressure would cause them to "bounce". We know the force of gravity and light pressure quite precisely, and the mechanics of compression waves, and so we can measure the ratio of mass that interacts with each force. And there's abot 5 times as much mass that reacts to gravity as mass that reacts to light pressure.

So, yeah, direct measurement of dark matter, and the exact measurement (which was 2 or 3 significant digits) was just what the dark matter hypothesis had predicted based on completely unrelated measurements of galaxy rotation speeds. Of course, that gives few clues about the nature of dark matter, but we know most matter in the universe is dark.

If there is to be a story [wikipedia.org] that resulted in the idea of missing matter, it should be attributed to Louise Volders discovery in the late 1950's and Vera Rubin's [wikipedia.org] additional work in the 1970's about the rotational speed of galaxies being faster than the observed mass of the galaxies--contradicting the acurate results observed when applying the same classical mechanics to the rotational speed of our solar system. Vera Rubin argued convincingly that Fritz Zwicky's [wikipedia.org] unrelated work [wikipedia.org] in the 1930's (for accurately cal

This or something similar seems to come up every time there is discussion of dark matter: "Dark matter was a lazy fix, instead physicists should realise theories were lacking and come up with a new one instead."

Physicists did see physics was incomplete, they did come up with a new theory: dark matter. Dark matter is a new theory and is a way of saying previously physics was incomplete. Physicists also came up with dozens of other theories, but in the end they haven't been doing as well at matching evide

This or something similar seems to come up every time there is discussion of dark matter: "Dark matter was a lazy fix, instead physicists should realise theories were lacking and come up with a new one instead."

He's probably posting from the USA, where everyone knows more than the experts.

Dark energy and dark matter are simply lazy science.The problem is that physics is wrong, or at least incomplete, not that there's some invisible force guiding matter to do strange things that leave only highly questionable evidence behind.

I think and hope above post was aiming for "funny", not for "troll"... And I certainly hope it was not aiming for "In*" moderation...

Dark matter does not interact electromagnetically. In other words is *can't* glow, it *can't* absorb light or microwaves or anything. We can see dark stuff, it blocks the CMB and other things. The fact that it does not interact except via gravity also means that is its very diffuse.

Consider a non interacting particle falling from 1 light year out towards the sun. It falls right thought and out the other side and comes to a stand still 1 light year away. It will do this forever since its does not interact with anything that can slow it down. Now consider a say consider about 9x the mass of the sun of these particles, they will always occupy a massive volume and hence be very very diffuse... but in the large scale (galactic) are a large effect with their combined gravity. The are Dark in the sense that they *only* interact via gravity.

They interact mostly only via gravity, even with each other. They are so diffuse or dilute that yes they interact with earth via gravity but not in a currently detectable way, but we have detected it around clusters, in particular the bullet cluster.

After reading the article, I wonder if there is not an inherent bias in the approach.

What they measured was the deflection of light from galactic sources that were 6 billion light years away. Using a method not described in the article, they were able to measure the amount of gravitational deflection between the light sources and our current observation position. I believe this is a composite measure of gravitational lensing.

I assume that their measurements are accurate. To my understanding, there is als

The amount of lensing measures the mass, since mass is what determines gravity.We don't know how many dark matter particles there are per kilogram, or anything like that, so we onlyknow the amount in one sense, mass, but that we do get from the lensing.

Pure bullshit. I have actually nothing to back that up, but the whole Dark Matter/Energy thingy sounds to weird to be true.More likely the physicists made some kind of wrong assumption early on, painted themselves in a corner and this is what they came up with.Same with the BB theory. They reckon that in the first few nano-secs there were no physical laws? Therefore the BB could do whatever it wanted to (if I'm allowed to add consciousness to that:) Forget rationality or believability.

Dark matter doesn't sound even remotely as weird as quantum mechanics. Yet quantum mechanics works incredibly well. You wouldn't have been able to type that comment without us understanding quantum mechanical effects that seem like magic. How uninituitive a theory sounds is a ridiculous judge of whether it is useful or accurate.

Does that map make it possible to calculate the temperature of the dark matter?

As far as I can understand it, if we know the spatial distribution we can infer how much kinetic enery each particle have. But I'm not so sure about that, there may be something I'm overlooking, or the current map may be less precise than the infered values from computer simulations. Do anybody knows the answer?

Yes, by measuring the infrared light (oh wait, it doesn't radiate, its dark) we can know the temperature. We can measure the volume, and the mass. To get the temperature we have to notice dark matter holding its up from gravity by pressure and not angular momentum, and also know the masses of the particles in dark matter, then PV=nRT.